GNETUM................................pptx

Gnetum

1. G NETUM Division: Gymnospermae Class : Gnetopsida Order : Gnetales Family : Gnetaceae

2. Gnetum has about 40 species, confined to the moist tropical forests of West Africa, India, South China and Malaysia. Five species occur in India. They are G.gnemon , G.contractum , G.latifolium , G.montamum G.ula . Young leaves, inflorescence fruits and seeds of G. gnemon are edible. Its bark yields a fibre of high tensile strength, ideal for paper pulp and fishing nets. Seed kernels of G.ula yield an oil, used for illumination, cooking and massage in rheumatism. The kernels of G.latifolium are edible and its bark is used for making nets and ropes.

3. Gnetum , represented by about 40 species is confined to the tropical and humid regions of the world. Nearly all species, except G. microcarpum , occur below an altitude of 1500 metres . Five species ( Gnetum contractum , G. gnemon , G. montanum , G. ula and G. latifolium ) have been reported from India Gnetum ula is the most commonly occurring species of India.

4. HABIT Majority of the Gnetum species are climbers except a few shrubs and trees. G. trinerve is apparently parasitic. Two types of branches are present on the main stem of the plant, i.e. branches of limited growth and branches of unlimited growth. Each branch contains nodes and internodes. In climbing species the branches of limited growth or short shoots are generally un-branched and bear the foliage leaves. The leaves (9-10) are arranged in decussate pairs

5. SPOROPHYTE Adult plant is sporophyte. It very much resembles angiosperms in appearance. The plant body is differentiated into root, stem and leaves. Root: Root system is profusely branched tap root system which does not grow very deep into the soil. Stem: Dwarf shoots of limited growth and long shoots of unlimited growth.

6. Leaves: Leaves are dorsiventral and are arranged in opposite decussate pairs. They closely resemble dicot leaves un having leathery texture broad lamina and reticulate venation.

7. Internal Structure Primary Structure of Root T.S of young root is differentiated into 3 regions Epiblema Cortex Vascular Cylinder

8. Epiblema , Cortex & vascular cylinder Epiblema is the outer most layer. Composed of compactly arranged cells. It gives out unicellular root hairs. Cortex is the multi-layered middle region. Made up of parenchyma cells. Cortical cells are oval or polygonal with a rich storage of starch grains. Cortex is internally limited by a single-layered endodermis. Vascular Region is externally bounded by the multi-layered pericycle . Vascular Cylinder is di-arch and exarch. There are 2 radially arranged Vascular Bundles separated from each other by parenchymatous tissue. Primary xylem has very few elements.

9. Anatomy of Gnetum In young Roots: Layers of starch filled cortical cells. 4-6 layers of pericycle ; primary xylem visible. Roots may be Di-arch and Ex-arch. In older Roots: Primary xylem indistinguishable due to secondary growth. Consist of tracheids , vessels and xylem parenchyma. Phloem consists of sieve tubes and phloem parenchyma. “Bars of sanio ” present in tracheids ; absent in vessels.

10. Secondary growth of root Secondary growth is of the normal type. Cambium are formed internal to the phloem groups and external to the xylem groups. Join to form a complete ring of cambium. Secondary xylem possesses vessels, tracheids and xylem parenchyma. Medullary rays transverse the secondary xylem, composed of parenchyma cells. Phloem is made up of sieve cells and phloem parenchyma. Periderm formation starts with the production of secondary vascular tissues. Phellogen originates from the outer cortical layer, produces phellem towards the outer side and phelloderm towards the inner side.

11. Primary structure of stem Stem has four major parts: Epidermis Cortex Vascular cylinder Pith

12. Primary structure of stem Epidermis Epidermis is the outermost layer. Made up of compactly arranged rectangular cells,with a thick cuticular covering.Epidermis is interrupted by sunken stomata.

13. Primary structure of stem Cortex Cortex is differentiated into three zones Outer chlorenchymatous Middle parenchymatous Inner sclerenchymatous zones.

14. Primary structure of stem Vascular Region Ring of primary vascular bundles. Vascular bundles are conjoint, collateral, open and endarch . Xylem consist of both tracheids and vessels. Phloem is made up of sieve tubes and phloem parenchyma.

15. Primary structure of stem Pith A parenchymatous pith is present at the centre. Its cells are polygonal in outline.

16. Leaves They resemble dicot leaves with cuticularised epidermis stomata on both side expect on veins. Mesophylls differentiated into single layered palisade, well-developed spongy parenchyma. Stone cell & latex tubes present in mid-rib region. Vascular bundles: Arch curve form in mid-rib region. Conjoint collateral type. Xylem: tracheids , vessels and xylem parenchyma. Phloem: sieve cells and phloem parenchyma

17. Reproduction in Gnetum: Reproduction is sexual. Gnetum is dioecious. The reproductive organs referred to as flowers are organized into inflorescences. Inflorescences are usually axillary in position but terminal inflorescences.

18. MALE STROBILUS: The male strobilus has a long slender axis. The male strobilus is branched and branching may be of several kinds. The axis bears 10-25whorl of bracts (collars). -About 12-25 male flowers are arranged in three to six rings above each collar.

19. In each collar, there are three to six rings of 12-15 or more male flowers and a single ring of 7-15 imperfect female flowers or abortive ovules is present above male flowers. A young strobilus is compact.

20. MALE FLOWER A male flower has two unilocular anthers enclosed in a small sheathing perianth. Its stalk is called antherophore which elongates at maturity. Consequently, the anther emerges beyond the collar through a slit in the perianth. The number of anthers in a male flower may also vary.

21. Female STROBILUS: It is similar to the male strobilus in the young stages. In a female strobilus, a ring of four to ten female flowers (ovules) is present above each collar. There is a total absence of any male flowers. Initially all the ovules look alike, but later only a few grow to maturity. The upper few collars lack ovules.